In situ ultraviolet-visible spectroscopy has been used to study diffusion of two azo-dyes in a CO2-swollen matrix of poly(methyl methacrylate) (PMMA). The diffusivity of both dyes can be tuned simply be changing the system pressure. Higher pressure of CO2 enhances diffusion of a dye in PMMA. The diffusion of dyes in CO2-swollen PMMA can also be influenced by specific interactions. The partitioning of the dyes between the polymer phase and the fluid phase was measured, and the partition coefficients are large (10(4)-10(5)). Thus, supercritical fluid dyeing is possible, although the solubility of the dyes in the fluid phase is low.